The present invention relates to insulating and leaktight coverings, and in particular coverings for industrial buildings.
These coverings fixed to a framework comprise a thermal insulation between a loadbearing element and a cladding. This insulation is principally realized by plates or panels laid touching one another. These plates are fastened firmly to the loadbearing element by a mechanical fixing means when a support consists of profiled steel sheets.
The mechanical fixing means consists of a screw, or a rivet or a bolt fastened firmly to the profiled steel sheet by drilling or by welding. At its upper part, a head of the screw, rivet or bolt has a washer of small dimensions, of a diameter of the order of 50 to 70 mm.
The cladding is fastened firmly to the insulating plates by adhesive bonding or by welding using a heat source, usually with a flame or air torch, over the entire surface of the insulating panels and/or at the level of the washers of the mechanical fixing means. The latter can be improved for welding by an appropriate surface coating or by a washer made from the same material as the cladding, and of larger dimensions, placed in between the metal washer and the underlying insulating panel.
A more recent technique provides for a first bed of foils constituting the lower part of the cladding to be unwound dry over the insulating panels. The mechanical fixing means then traverse the foil bed and the insulating panels. In the case of panels sensitive to the flame of a torch, a prior heat screen can be employed on the insulator. The upper part of the cladding is then adhesively bonded or welded to the lower part comprising its visible washers.
Another technique provides mechanical fixing means at the level of lap joints of the widths of the cladding. A lapped part is adhesively bonded or welded, on the one hand, to the edge of the adjacent width and, on the other hand, to the small washers of the fixing means.
The wind creates considerable localized compression and suction forces on the covering (vortices, shielding effect behind a wall or a salient part of the roofing). The forces are exerted on the outer surface and hence on the cladding, which ultimately stresses the loadbearing element and the structure of the building.
In the area lying between the cladding and the loadbearing element, these forces create, at the level of one or more fixing means, tear-away forces perpendicular and parallel to the covering surface. These latter forces are more substantial the greater the spacing between the fixing devices. They can result in the cladding ripping at the head of the fixing means and/or the fixing means being torn away at the level of their connection to the framework or the loadbearing element under a torque or traction effect.
For these two latter techniques, at least one sheet of the cladding is pierced by the mechanical fixing means.
In all cases, in order to resist the suction forces created by the wind, the French standards defined within the D.T.U. 43.3 and common practice provide for a minimum of five fixing means per m.sup.2 of roofing, based on the fact that a fixing means resists a tear-away force of approximately 900N.
A rupture occurs at the level of the plane of the adhesive bonding of the cladding to the washer, or as a result of the head of the fixing means becoming dislodged and passing through the washer, or as a result of the fixing means being torn away through the profiled steel sheet. The values of the rupture are relatively homogeneous, of the order of 900 to 1300N.
The large number of fixing means makes implementation lengthy and expensive. Furthermore, the performance of the cladding is considerably diminished at the level of the fixing means because the cladding is partially pierced, or because it can be punched by the head of the fixing means passing through the washer, or alternatively torn at the periphery of the washer when the latter is locked, redundantly, on the rod of the, fixing means, as described in French Patent 1,522,378. Such phenomenon favors the breaking of the weld between the bolt and the profiled steel sheet. These disadvantages are considerably amplified when the insulating panels are compressible but elastic.